1. Technical Field
Some embodiments of the present invention relates to a micro-electro mechanical system (MEMS) switch and a manufacturing method thereof.
2. Related Art
A MEMS switch is a switch having a minute structure formed on a substrate made of semiconductor or the like by using semiconductor manufacturing technologies. The MEMS switch has a fixed electrode fixed on the substrate and a movable electrode having a structure such as a cantilever beam, a doubly-supported beam, a diaphragm and the like. An on/off action of the MEMS switch is performed by utilizing an electrostatic force or the like.
JP-T-2005-512830 is a first example of related art and JP-A-2005-124126 is a second example of related art. The first example discloses a MEMS switch having a movable electrode (referred to as “a flexible member” in the first example) which is made of an alloy in order to reduce an electric resistance generated when the MEMS switch is conductive. The second example discloses an application example of a MEMS switch in which the MEMS switch is used as a selector switch for selecting a radio-frequency band. In this case, an internal loss is smaller compared with a switch using a varactor diode or the like, and it is possible to obtain a high Q value (which represents resonance sharpness).
When the MEMS switch fabricated by using the technology according to the first example is applied to for example a voltage-dividing circuit, an external resistance is further needed for the voltage division since the resistance of the MEMS switch is too small. This means that the number of the components provided around the MEMS switch is increased. Accordingly, an area of a chip is expanded and a temperature difference within the chip increases. Consequently, temperature differences among resistances used in the voltage-dividing circuit become larger and differences in the resistance values of the resistances increase due to the temperature coefficient of each resistance. This deteriorates the accuracy of the voltage value that can be obtained from the voltage-dividing circuit.
When the MEMS switch is applied to a radio-frequency band by using the technology according to the second example and used to form for example an attenuator, the area where a chip occupies is increased in the same manner as the above-described case of the first example. The wave-length to which the attenuator can accommodate increases as the size of the chip which depends on the area of the chip increases. An operational frequency therefore decreases since it is proportional to the inverse of the wavelength, and this limits the operational bandwidth in a high-frequency band.
Where the MEMS is formed from a low resistance material such as alloys, the Q value which presents resonance sharpness increases and it affects largely even with a slight mismatch of the impedance. More specifically, the movable electrode of the MEMS switch serves as a short-stub when the movable electrode of the MEMS contacts with the fixed electrode. The movable electrode of the MEMS switch serves as an open-stub when the movable electrode do not contact with the fixed electrode. The amount of reflection from the short-stub or the open-stub will become large when the Q value is high, making the operation in the high-frequency band unstable.